Origin of Tin Mineralization in the Sullivan Pb-Zn-Ag Deposit, British Columbia: Constraints from Textures, Geochemistry, and LA-ICP-MS U-Pb Geochronology of Cassiterite

Abstract

Textural, geochronological, and geochemical data are presented here for cassiterite from the giant (149.7 million tonnes [Mt]) Mesoproterozoic Sullivan Pb-Zn-Ag deposit, which has been subjected to several tectonothermal events. These data provide constraints on the age and origin of the tin concentrations and new insights into related base metal mineralization. Sullivan is rare among sediment-hosted, stratiform Pb-Zn-Ag deposits in having high tin contents in ore (up to 2.5 wt %; avg 310 ppm Sn). Cassiterite occurs in all facies of this deformed and metamorphosed deposit, including (1) high-grade veins with arsenopyrite and pyrrhotite, (2) bedded Pb-Zn-Ag ores, (3) massive pyrrhotite, (4) footwall and hanging-wall tourmalinites, and (5) other altered wall rocks. New in situ U-Pb dates for Sullivan cassiterite obtained by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are modeled by a multicomponent-based algorithm that yields three age peaks: 1475 ± 4 Ma (51% of the data), 1366 ± 10 Ma (25%), and 1074 ± 7 Ma (24%). These dates are attributed, respectively, to primary tin mineralization at ca. 1475 Ma, the East Kootenay orogeny at ca. 1370 to 1300 Ma, and the Grenvillian orogeny at ca. 1100 to 980 Ma. Based on the presence and local abundance of cassiterite in all ore and ore-related rocks at Sullivan, the U-Pb date of 1475 ± 4 Ma reported here represents the first direct age for ore mineralization in the deposit. Occurrence of texturally discordant rims on Sullivan cassiterite grains having U-Pb dates coeval with the East Kootenay and Grenvillian orogenies suggests that these young dates reflect dissolution-reprecipitation processes associated with channelized metamorphic fluid flow. LA-ICP-MS U-Pb dates obtained on low-U (<10 ppm) cassiterite also indicate that U-Pb dates for cassiterite from other metamorphosed deposits should be viewed with caution and not assumed to record an age of primary tin mineralization. Aqueous transport conditions for tin are evaluated to gain insights into the cassiterite mineralization at Sullivan. Based on fO2-pH topology of aqueous tin species at 250°C, tin transport was dominated by an SnCl3− complex at fO2 of about –40 and pH of <4.0, conditions that were constrained, respectively, by widespread occurrence of pyrrhotite in deep footwall siliciclastic metasedimentary rocks of the host Aldridge Formation and by release of CO2 from shallow mafic sills and resulting formation of carbonic acid in condensed brine. The low fO2 value also reflects inferred production of CH4 from heating of organic matter in the sediments during emplacement of these sills. Based on a fluid pH restriction of <4.0 and a requirement for sparse or no K-feldspar in the source, the tin likely derives from previously altered Lower Aldridge strata. This model relies on the early diagenetic dissolution of K-feldspar from these sediments by basinal brines, followed by interaction with a later, more acidic hydrothermal fluid generated during the emplacement of large mafic sills in the shallow subsurface that leached tin from accessory minerals such as titanite in siliciclastic sediments of the Lower Aldridge Formation. Mass balance calculations suggest that derivation of the tin from this sedimentary source (avg 2.0 ppm Sn) required ~40 km3 and a cylinder diameter of 3.2 km (height 5.0 km) in order to supply the 0.1 Mt of tin contained in the deposit. The presence of mafic sills in the footwall of several other tin-bearing, sediment-hosted, stratiform Pb-Zn-Ag deposits and in modern, tin-rich, sediment-hosted sulfide deposits in the northeast Pacific Ocean suggests that siliciclastic marine basins that contain mafic sills—with or without stratiform sulfide deposits—should be evaluated for possible tin mineralization.

Caledonian Tin Mineralization in the Jiuwandashan Area, Northern Guangxi, South China

“…In addition, cassiterite grains exhibit distinct oscillatory growth bands in back-scattered electron images (Figure 4). Our results show that some cassiterite crystallized from hydrothermal fluids and has low U contents (U < 10 ppm), and, thus, the cassiterite rims record the timing of fluid dissolution and recrystallization, whereas the cores record the age of primary cassiterite formation [35]. There are no obvious differences between the U-Pb ages of cassiterite cores and rims.…”

Perspectives on premetamorphic stratabound tourmalinites

“…This hypothesis can be tested via mass-balance calculations for distinguishing between marine and non-marine sources of boron. For example, Slack et al (2020) used this approach in evaluating the source of the ~52 Mt of boron contained within tourmalinite alteration zones of the Sullivan Pb-Zn-Ag orebody in Canada (Fig. 4).…”

Synsedimentary to Diagenetic Cu±Co Mineralization in Mesoproterozoic Pyritic Shale Driven by Magmatic-Hydrothermal Activity on the Edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: Evidence from Sulfide Re-Os Isotope Geochemistry

“…1,468 Ma gabbroic sills found in rift-facies marine turbidites in the main Belt-Purcell Basin, in particular within footwall sediments of the ca. 1,475 Ma synsedimentary Sullivan deposit (U-Pb TIMS cassiterite geochronology, [16]) hosted in the Lower Aldridge Formation, southeastern British Columbia, Canada (Figure 1) [1,17,18]. The Newland Formation is interpreted as (1) being slightly younger than the Prichard Formation (i.e., lateral equivalent to the Lower Aldridge Formation) and (2) older than the >1,454 Ma Revett Formation (SHRIMP U-Pb geochronology of bentonite) [19] in northwestern Montana and Idaho in the main Belt-Purcell Basin ( [20]; detailed stratigraphic correlations in [7], their Figure 2).…”

“…In the Belt-Purcell Basin, metalliferous hydrothermal and magmatichydrothermal fluids circulated in the main branch of this basin at least from the time of Zn-Pb-Ag mineralization in the Sullivan deposit at ca. 1,475 Ma (U-Pb TIMS cassiterite age) [16] to the time of magmatic-hydrothermal Cu-Co-Au mineralization at ca. 1,349 Ma in the Idaho cobalt belt (Re-Os isochron age of cobaltite) [39].…”

“…NJS and RAC designed the study during field season 2015 while hosted by Tintina Montana Inc. and its Senior Vice President JZ. NJS carried out petrographic observations, 16 Lithosphere…”